Protective device encapsulants, e.g., conformal coatings were originally developed to protect sensitive electronic assemblies from the harsh environments experienced in military, aerospace and marine domains. However, as the level of integration has increased in the electronic industry, e.g., with SMT and finer lead pitches associated with VLSI circuitry, the use of and need for adequate protective coatings has spread into a variety of commercial (as well as military) applications.
The continuing miniaturization of electronic systems has resulted in the integration of direct-chip-attach (or DCA) and chip-scale-package (or CSP) technologies. These technologies allow for the miniaturization of electronic systems by means of eliminating large chip packages. However, improved protective coatings and associated application and/or removal techniques are needed to provide such modules with the reliability and field performance of packaged ICs.
The ideal encapsulant should have properties that allow for easy application and removal, low cure temperature, temperature resistance, humidity resistance, and long potlife. Of the presently used materials (e.g., acrylics, polyurethanes, epoxies, silicones, polyimides, and polyparaxylylene) none exhibits all of the ideal properties for a suitable encapsulant. The tradeoff is typically between the ease of application and processing on the one hand versus protective capability and environmental stability on the other hand. For example, acrylics are generally easy to apply and remove but typically exhibit low temperature and humidity resistance. Conversely, polyimides, generally have high temperature and humidity resistance but are difficult to apply, require a high cure temperature, and have short pot life. Another example is a material known as RTV, which has been used to coat chip-on-board systems. Unfortunately, however, RTV is reactive and cannot be directly applied without extensive board preparation. Epoxy coatings may be applied as glob-top encapsulants, but because epoxy has a significantly different thermal expansion coefficient than most printed circuit board materials, the curing process and environmental thermal cycling can result in unacceptable mechanical stress and failure at the component or board level. In addition, epoxies are not generally acceptably reovable for rework. Polyurethane has also been used for such coatings, but is unstable under high temperature and humidity. Significantly, most of these currently employed coatings are difficult to remove, making repair problematic and, in some cases, practically impossible.
Accordingly, what is needed is an improved encapsulant for protectively coating an electronic device. Moreover, what is needed is a solution for reworkably encapsulating an electronic device module.